Adhesives used for the transdermal drug delivery system include three categories: rubber adhesives (polyisobutylene rubber, styrene-isoprene copolymer etc.), silicon adhesives and acrylic adhesives.
Rubber adhesives have the advantage in that it is easy to control their physical properties by using various molecular weights of polymers and additives. However, rubber adhesives are highly hydrophobic and have poor adhesive properties in moisture rich environments, i.e. they adhere poorly to skin which is wet with perspiration.
In order to solve the above-mentioned problems, rubber adhesive compositions comprising a rubber polymer as a main component and a water-soluble hydrocolloid such as pectin, gelatin, and carboxymethylcellulose have been disclosed in U.S. Pat. Nos. 3,339,549, and 4,551,490 and European Patent 591 898. U.S. Pat. No. 5,176,490 also discloses a hydrocolloid adhesive composition comprising a base mainly composed of a hydrophobic polymer, water and a hydrophilic polymer. This adhesive, reportedly, has satisfactory water absorbability. However, the properties of the adhesive compositions mentioned above are merely suitable for plaster formulations that have a relatively large size, e.g. surface area, and thickness. When the adhesive layer absorbs water, the adhesive strength is significantly reduced and the plasters fail to remain attached. In addition, conventional plasters containing water-soluble polymers have poor adhesion when applied onto curved body parts for functional periods of time due to the thickness of the adhesive layer. Furthermore, conventional water-soluble plasters cannot be loaded with sufficient amounts of active ingredients because most drugs are poorly soluble in water. Therefore, plaster formulations are generally not suitable to be used as transdermal drug delivery devices.
Use of cross-linked polymers with hydrocolloids has been tried in order to maintain adhesive strength when the skin is wet. However, the results were not satisfactory in maintaining the adhesive strength. EP 0 343 807 A2 discloses a wound dressing comprising 30-65 wt. % of polyisoprene, 10-30 wt. % of a polyvinylpyrrolidone, 2-20 wt. % of a modified starch, 2-20 wt. % of pectin, 0.1-10 wt. % of an acrylic polymer, and 0-1 wt. % of fiber to control the absorption of water. In addition, EP 0 591 898 A1 discloses adhesive compositions for wound dressings comprising at least one hydrocolloid, hydrophobic unsaturated aliphatic polymers, and adhesive enhancers. However, the adhesive compositions mentioned above are comprised mainly of a rubber adhesive, as the adhesive component, which has high hydrophobicity. Therefore, they cannot be used for efficiently delivering active ingredients in their salt forms. In addition, they fail to maintain adhesive strength on skin that is wet.
Acrylic adhesives offer a number of advantages. For example, most drugs are more readily soluble in acrylic adhesives than in silicones and rubbers. Acrylic adhesives are highly compatible with or soluble in the commonly used skin penetration enhancers. In addition, the chemical properties of acrylic adhesives are easily manipulated to permit the preparation of both polar and non-polar adhesives. However, despite these advantages, acrylic adhesives are poorly soluble in water and have poor adhesion to skin which is wet with perspiration. Therefore, there is need for devices with improved adhesion properties, especially for delivering highly hydrophilic drugs.